The present invention relates to apparatus and to a method of combining high-performance liquid chromatography with capillary gas chromatography. More particularly, the invention is directed to a method and apparatus for on-line coupling of liquid and gas chromatography columns in which there is direct, uninterrupted transfer of sample containing eluent from a liquid chromatographic analytical system to a gas chromatographic system.
In the general area of multidimensional chromatography, the introduction of selected fractions from a liquid chromatographic analytical system into a gas chromatographic system has been described. (K. Grob, Jr., et al., J. Chromatography 295, 55-61 [1984]). However, for the most part, such analyses have been carried out utilizing off-line techniques requiring collection and reinjection of the separate fractions, or by on-line procedures using conventional High Performance Liquid Chromatographic Columns (HPLC), where only a fraction of the separated peak could be introduced into the gas chromatographic equipment.
Efforts to increase the theoretical plate count and the speed of analysis in HPLC have been directed to reducing the particle size of the chromatographic support to a few microns, using open tubular columns analogous to capillary gas chromatography, and reducing column diameter in microbore and, more recently, using packed capillaries.
Among the advantages realized in utilizing packed capillaries for HPLC are the higher total efficiencies obtained by using longer columns, and the reduced eluent consumption. The latter feature permits the use of exotic solvents and the interfacing to detectors such as flame based and mass spectrometers.
The prior art does describe the use of conventional liquid chromatographic columns which have been coupled to gas chromatographic assemblies. However, the relatively fast flow rates (that is, the large volumes) used permitted only fractions of a peak to be introduced to the gas chromatographic assembly, or only the characterization of well resolved components, for example, one or two peaks in a mixture.
In spite of the widespread interest in conducting sequential, uninterrupted analyses utilizing the combination, in series, of a liquid chromatographic system followed by a gas chromatographic system, no completely satisfactory apparatus or technique has heretofore been achieved. It is, therefore, a principle aim of the present invention to provide an improved apparatus and method whereby the deficiencies and shortcomings of prior art techniques and equipment may be overcome.
Terms
A low flow rate liquid chromatography column means a column whose effective operating flow rate allows quantitative transfer of the component(s) of interest into a gas chromatography column through a capillary interface. Low flow rate liquid chromatography columns useful in the invention are generally capillary columns of an inner diameter of about 1 mm or less. Under careful flow conditions it is contemplated that larger columns can be employed for the purposes of the invention, e.g., liquid chromatography columns having an inner diameter of generally about 2 mm or less.
Quantitative transfer means introduction of the total component(s) of interest in the mixture eluting from the liquid chromatography column to the gas chromatography column.
Effective operating flow rates mean a flow rate not greater than that which would not allow effective trapping of the component(s) of interest at the head of the gas chromatography column.
Effective trapping means and is shown by not obtaining detrimental peak shapes, band broadening, or significantly diminished resolution which occurs because of excess eluent volume which carries the component(s) of interest from the interface into the gas chromatography column to an extent that it creates band broadening, diminished resolution, and/or detrimental peak shape shown by excessive nongaussian peak character.
Detrimental peak shape, band broadening, and diminished resolution mean those conditions which produce unnecessary merged peaks, unnecessary nongaussian shapes of peaks which are difficult to quantitate, or peaks that are unnecessarily broadened to the point that quantitation at lower levels based, e.g., on peak height, becomes difficult or impossible.